This invention is concerned with circuitry for the efficient delivery of relatively high-power radio-frequency (RF) energy to a load, such as a plasma chamber or other device. The invention is more particularly concerned with a harmonic filter which absorbs energy at frequencies that vary from a predetermined RF frequency, and more specifically with a filter which protects the RF generator from energy coming back from the plasma chamber at harmonics and subharmonics which may result from application of the RF energy to a non-linear plasma load.
In the art of plasma deposition or sputtering, for example, the process is driven by radio frequency energy typically provided at a frequency of 13.56 MHz at levels up to several kilowatts. Typically, there is an RF generator coupled to a plasma chamber with a matching network interposed between them to match the source plasma generator to the RF generator source impedance which is typically 50 ohms.
Because the plasma does not behave like a linear, ohmic resistance, the application of RF energy to the plasma generator produces out of band energy which can be at multiples of the source frequency (harmonics), or at fractions thereof (subharmonics).
An RF energy delivery system comprising an RF generator, a matching network, and a load is described in U.S. Pat. No. 4,679,007, and generally represents state-of-the-art methodology. A harmonic filter can be interposed in between the generator and the matching network to deal with the problem of out-of-band signals generated by the non-linearity of the plasma load. Previous attempts to do this have generally involved reflective type filters, which reflect rather than absorb harmonics. These previous systems have not dealt adequately with both harmonics and subharmonics. Generally filters of this type have involved a low pass filter followed by a high pass filter in combination with a resistive termination.
Dissipative filters have often been employed in communications work, for example, as a narrow bandpass I.F. filter after the first down converter of a receiver front end. Dissipative filters are employed to improve performance, where it is needed to provide proper controlled termination to the out of band signals. However, dissipative filters have not been favored because they do not offer the sharp attenuation slope of an equivalent lossless filter. Consequently, because of energy dissipation problems, circuit designers have been reluctant to interpose a filter of this type in an RF delivery system where the RF power can be several kilowatts.